The reaction of a carbonyl compound with hydroxylamine is a major process for the synthesis of the corresponding oxime compound. For example, cyclohexanone oxime is a key intermediate for producing ε-caprolactam; while ε-caprolactam is an important raw material for organic chemical industries, mainly used as a monomer for synthetic fibers and engineering plastics (e.g., nylon-6). About 91% of caprolactam are produced industrially through a technique route with cyclohexanone oxime as intermediate product, in which cyclohexanone oxime is produced by the reaction of cyclohexanone with hydroxylamine (used in its sulfate or phosphate form). This process for the production of cyclohexanone oxime has a complex technology with multiple process steps and high investment in equipment, and it also has a problem in corrosion and pollution due to use or production of NOx, SOx and the like.
In the early 1980's, in U.S. Pat. No. 4,410,501, Taramasso (Italy) disclosed a novel type of catalytic material—titanium silicalite having an excellent function for selective oxidation of hydrocarbons, alcohols, phenols and the like (EP 0230949, U.S. Pat. No. 4,480,135, U.S. Pat. No. 4,396,783). It has been commercialized to use it for the preparation of catechol and hydroquinone by the selective oxidation of phenol with hydrogen peroxide.
EP 0208311, EP 0267362, EP 0496385, EP 0564040 and so on sequentially disclose a process for preparing cyclohexanone oxime in one step by ammoximation of cyclohexanone with ammonia and hydrogen peroxide catelyzed by titanium silicalite. This novel process features in mild reaction conditions, high yields of desired products, more efficient process, lower investment in equipment, reduced amount of wastes and environmental friend.
Furthermore, applications of other titanium-containing catalytic materials in ammoximation have also been reported. For example, EP 0347926 discloses that the ammoximation of cyclohexanone is carried out by using a catalyst in which titanium dioxide is dispersed on silica, both J. Le. Bars et al., Appl. Catal. A 136(1996) p.69 and P. Wu et al., J. Catal. 168 (1997) p. 400 report other types of Ti-containing crystalline silicate (e.g. Ti-ZSM-48, Ti-β, Ti-MOR and the like) which are used in ammoximation of a variety of aldehyde or ketone compounds.
The reaction of a nitrogenated basic compound with hydrogen peroxide is a major process for the synthesis of the corresponding hydroxylamine. In U.S. Pat. No. 4,918,194 and U.S. Pat. No. 5,320,819, it is reported that oxidation of a nitrogenated basic compound (e.g. secondary amines, ammonia and the like) is carried out by using a titanium silicalite or an amorphous titanium-containing catalyst.
With catalytic reactions being studied increasingly and intensively, more attention has been increasingly paid to the problem on stability of titanium-containing catalysts, especially titanium silicalites , in the ammoximation reaction of cyclohexanone for the preparation of cyclohexanone oxime. EP 0496385 reports that it is necessary to remove the deactivated catalyst periodically, which is to be replaced by a fresh catalyst make-up in order to maintain the desired catalytic activity during the reaction. How to improve the stability of titanium silicate molecular sieve has become a focus.
U.S. Pat. No. 4,794,198 discloses a process for pre-treating catalyst to increase the selectivity and improve the stability of catalyst. P. Roffia et al., Stud. Surf Sci. Catal. 55(1990) p.43 proposes that the catalytic reaction rate can be increased and the non-catalytic reaction thus can be minimized by optimizing the process conditions, e.g. selecting an appropriate solvent and increasing the catalyst concentration and reaction temperature. However, the improvement of the catalyst stability thus achieved is limited, since it has the problem concerning how to utilize and regenerate the deactivated catalyst due to the high cost of the titanium silicalite.
In general, there are two methods for regenerating the deactivated catalyst: washing with a solvent and calcining. In the book Selective Oxidation by heterogeneous Catalysis (2001), p.112, it is indicated that three main deactivation processes of titanium silicalites in the ammoximation of cyclohexanone: (1) slow dissolution of the framework (silicon) with accumulation of Ti on the external surface of the remaining solid, (2) direct removal of Ti from the framework and (3) pore filling by by-products. The book further points out that, the deactivation by pore filling can be partially eliminated by washing the catalyst with t-butanol. The regeneration effect is poor.
CN1302693A discloses a regeneration process of subjecting the catalyst, previously calcined, to a treatment in an aqueous medium with hydrogen peroxide in the presence of inorganic fluorinated compounds, subsequent to thermal treatment. This process is illustrated with an example related to a regeneration procedure of a catalyst deactivated in the ammoximation of cyclohexanone for preparing cyclohexanone oxime, wherein the catalytic activity of the regenerated catalyst can be recovered up to 84% relative to the activity of fresh catalyst. It also points out that if a catalyst is regenerated by thermal treatment only (calcined at 550° C.), the catalytic activity of the regenerated catalyst can be recovered to merely 31% relative to the activity of fresh catalyst. Anyhow, the catalytic activity of the regenerated catalyst obtained by this process is still not fully recovered to the level of fresh catalyst. Moreover, in this process, fluorinated compounds, are used in the chemical treatment thereof, which tend to be toxic and harmful.
CN 1290194A discloses a method of regenerating a supported catalyst covered with gold particles, based on titanium dioxide or titanium dioxide hydrate. The invention is characterised in that the catalyst is regenerated by contacting it with water, a diluted acid or a diluted hydroperoxide solution, to restore its catalytic activity. The catalyst used in this patent is prepared with a “deposition-precipitation” method and used for oxidizing unsaturated hydrocarbons in the gas phase. The dilute acid has a pH value of 4˜7.5, preferably 5.5˜6 and is preferably dilute H2SO4 or HF. Taking the oxidation of propene as an example, the catalytic activity of the catalyst regenerated according to the method of this patent can be recovered up to 80% relative to the activity of fresh catalyst It is impossible to recover the catalytic activity of the regenerated catalyst to the level of fresh catalyst by this method. Furthermore, the patent does not mention the stability of the regenerated catalyst